Known systems which form SiO2/TiO2 glass are distinguished by their good thermal properties, their high refractive index and their low coefficient of thermal expansion. The low coefficient of expansion is of importance for the production of optical components in which dimensional accuracy must be guaranteed over a wide temperature range.
A known process for the production of binary SiO2/TiO2 glasses is by melting. Disadvantages are on the one hand that the melting temperature of 1,700° C. is very high, and on the other hand phase separation and devitrification very readily occur during cooling of the glass melt.
Another known method which is most frequently employed for the production of SiO2/TiO2 glasses is the sol-gel process. In many cases this comprises hydrolysis and condensation of organometallic compounds.
U.S. Pat. No. 4,278,632 describes the production of SiO2/TiO2 glasses by reaction of alkoxides of silicon and titanium or the partly hydrolysed alkoxides in the presence of water. After the hydrolysis the material is dried and optionally sintered.
U.S. Pat. No. 4,786,618 describes a process for the production of SiO2/TiO2 glass of low coefficient of thermal expansion (ULE=ultra-low expansion) by means of an alkali metal silicate solution and colloidal TiO2 having a pH of >9. The glass obtained is free from inhomogeneities and has a lower coefficient of expansion than silica glass. The glasses produced in this way comprise between 3 and 10 wt. % of TiO2.
A disadvantage of the sol-gel process is that only low green densities can be obtained. High shrinkage rates consequently occur during drying and sintering. These process steps must be carried out very slowly, often over days and weeks, in order to avoid cracking.
It is furthermore known to use gas phase deposition processes, such as, for example, the so-called CVD process (CVD=chemical vapour deposition) for the production of SiO2/TiO2 glasses. The temperatures in these processes are between 200 and 2,000° C. The production of a clear glass with up to 16 wt. % TiO2 is possible by this method.
U.S. Pat. Nos. 2,305,659 and 5,970,751 disclose combustion of precursors of silicon and titanium together in a flame to form particles which are in general called glass black or soot. The soot particles are deposited on a carrier and the porous body obtained in this way is then converted at high temperatures of about 1,500° C. to an opaline glass body, which in turn sinters at even higher temperatures to give a transparent glass body. The glass bodies (boules), which are as a rule very large, are further worked up, for example by cutting, into smaller pieces, such as, for example, for the production of lenses. They can also be combined to give larger optical bodies.
A disadvantage of the soot process is that three-dimensional shaped glass articles cannot be obtained directly from the process with an acceptable economic outlay. To obtain glass with various shapes, mechanical working is necessary (W. T. Minehan, G. L. Messing and C. G. Pantano, Titania-silica glasses prepared by sintering alkoxide derived spherical colloids. J. Non-Cryst. Solids 108 (1989) 163-168).
Because of the relatively low rate of deposition, the yield in the processes of deposition from the gas phase is rather low.
A composition of the SiO2/TiO2 soot particles is given, for example, in U.S. Pat. No. 5,180,411. According to this patent, three different forms are assumed to be present. On the one hand agglomerates of mixed oxide particles of SiO2 and TiO2 with a particle diameter of approximately 0.1 to 0.3 μm are present. In addition there are fine anatase TiO2 particles on the surface of the SiO2/TiO2 agglomerates. Finally, larger anatase crystals with a diameter of between 0.2 and 1 μm are present in the soot. It is difficult and requires high process technology control to produce a uniform glass body from this non-uniform powder, which additionally can also be obtained in a varying composition.
Since in gas phase deposition by means of the soot process large amounts of SiO2/TiO2 soot which has not been deposited are obtained as a waste product, attempts have been made to employ this waste product elsewhere. WO 00/48775 describes a process for the production of honeycomb structures of glass by extrusion of SiO2/TiO2 soot. In this, the soot is processed in the presence of organic additives to give a paste, which is first extruded to a green body, then freed from the organic constituents by heating and finally sintered. A disadvantage of this process is the use of organic binders, which must be removed by combustion. Furthermore, shaped SiO2/TiO2 glass articles of optical quality cannot be produced.